1. Field of the Invention
The present invention relates to a polarizing element for use in an optical display apparatus such as a transmission-reflection-type liquid crystal display. In particular, the present invention relates to a polarizing element adapted to enhance contrast so as to provide improved displaying quality to an optical display apparatus, and to an optical display apparatus including such a polarizing element.
2. Description of Related Art
A transmission-reflection-type liquid crystal display has conventionally been known as a liquid crystal display that can be used with extraneous light in a reflection mode and with a backlight in a transmission mode. A semi-transmission reflector is usually used in such a transmission-reflection-type liquid crystal display as a reflecting plate for reflecting extraneous light. This semi-transmission reflector reflects extraneous light when the display is used in a reflection mode, and, when the display is used in a transmission mode, transmits a backlight emitted from a surface light source provided at its rear.
In recent years, a layer having a cholesteric regularity (cholesteric layer) has come into use as the above-described semi-transmission reflector. A cholesteric layer has the property of selecting polarized light (the property of separating polarized light), by which incident light is separated into a component circularly polarized in one direction and a component circularly polarized in the opposite direction, on the basis of the physical arrangement (planar arrangement) of molecules in the cholesteric layer. Light entering into the cholesteric layer along the helical axis of the planar arrangement is split into a right-handed polarized component and a left-handed polarized component, and the cholesteric layer reflects one of these circularly polarized components and transmits the other one. This phenomenon is widely known as dichroism. If the direction of optical rotation of a circularly polarized component is properly selected in terms of incident light, a component circularly polarized in the same direction as that of the helical axis of the cholesteric structure of a cholesteric layer is selectively reflected or transmitted.
In the case where a cholesteric layer is used in a display as the semi-transmission reflector, a circular polarizing plate (elliptical polarizing plate) is provided at the rear of the cholesteric layer, i.e., on the surface light source side, so that light that passes through the cholesteric layer when the display is used in a reflection mode, in which extraneous light is used, will not return to the observer side. (Please note that there are no real circular polarizing plates that can produce a completely circular state of polarization in a practical level; and thus, all circular polarizing plates that cannot produce a completely circular state of polarization, including those polarizing plates, that are conventionally called circular polarizing plates, are referred to as xe2x80x9celliptical polarizing platesxe2x80x9d in this specification.)
FIGS. 8A and 8B are schematic views illustrating the operation of a transmission-reflection-type liquid crystal display, and correspond to a transmission mode and a reflection mode, respectively.
As shown in FIGS. 8A and 8B, the cholesteric layer for use in a transmission-reflection-type liquid crystal display has the properties of not perfectly (100%) reflecting a specific circularly polarized component of visible light but transmitting this component to some extent. The ratio of transmission to reflection may be established so that it falls in the range of 95:5 to 5:95.
Specifically, when the display is used in a transmission mode, the cholesteric layer reflects a part (L3) of the right-handed circularly polarized light (L2) and transmits the remainder (L1) of the same, as shown in FIG. 8A. On the other hand, when the display is used in a reflection mode, the cholesteric layer reflects a part (L3) of the right-handed circularly polarized light (L2) and transmits the remainder (L1) of the same, as shown in FIG. 8B.
In general, however, the aforementioned conventional liquid crystal display of transmission-reflection type has such a problem that contrast in a transmission mode (see FIG. 8A), in which a backlight is used, is low. Therefore, there have been strong demands for elucidation of the cause of this problem and improvement in contrast.
To meet these demands, the inventor has made earnest studies, and, as a result, has found that contrast in the transmission-reflection-type liquid crystal display depends largely upon how the cholesteric layer and the elliptical polarizing plate, which is provided on the surface of the backlight unit, are laminated to each other. In particular, if the state of polarization before and after the cholesteric layer (L2, L1) is disordered in a transmission mode (see FIG. 8A), the right-handed circularly polarized light is not fully extinguished by a right elliptical polarizing plate, which is provided on the observer side, when a liquid crystal displaying element is switched on; and light leakage thus occurs to lower the quality of black indication.
The present invention was accomplished on the basis of the above-described finding. An object of the present invention is to provide a polarizing element adapted to enhance contrast so as to attain improvement in the displaying quality of an optical display apparatus, and an optical display apparatus including such a polarizing element.
A first aspect of the present invention is a polarizing element comprising a cholesteric layer that selectively reflects either one of right-handed and left-handed circularly polarized components of incident light; and an absorptive elliptical polarizing plate that is laminated to the cholesteric layer. The polarizing plate transmits elliptically polarized light that contains as its main component the circularly polarized component having the direction of optical rotation that is identical to that of the circularly polarized component selectively reflected by the cholesteric layer. The cholesteric layer and the absorptive elliptical polarizing plate are laminated to each other so that the direction of the major axes of liquid crystalline molecules aligned on the absorptive-elliptical-polarizing-plate-side surface of the cholesteric layer can form, with the direction of the major axis of elliptically polarized light produced by the absorptive elliptical polarizing plate, an angle that falls in a specified angle range centering around 90xc2x0.
A second aspect of the present invention is an optical display apparatus comprising a displaying element for displaying an image by modulating light; a surface light source provided on the back side of the displaying element; and a polarizing element provided between the displaying element and the surface light source. The polarizing element includes a cholesteric layer that selectively reflects either one of right-handed and left-handed circularly polarized components of incident light emitted from the surface light source; and an absorptive elliptical polarizing plate that is laminated to the cholesteric layer and that transmits elliptically polarized light that contains as its main component the circularly polarized component having the direction of optical rotation that is identical to that of the circularly polarized component selectively reflected by the cholesteric layer. The cholesteric layer and the absorptive elliptical polarizing plate being laminated to each other so that the direction of the major axes of liquid crystalline molecules aligned on the absorptive-elliptical-polarizing-plate-side surface of the cholesteric layer can form, with the direction of the major axis of elliptically polarized light produced by the absorptive elliptical polarizing plate, an angle that falls in a specified angle range centering around 90xc2x0.
In the above-described first and second aspect, it is preferable that the absorptive elliptical polarizing plate be composed of a retardation layer provided on the cholesteric layer, and a linear polarizing layer provided on the opposite side of this retardation layer from the cholesteric layer. It is also preferable that the specified angle range be 90xc2x0xc2x122.5xc2x0, more preferably 90xc2x0xc2x1150xc2x0. Further, the cholesteric layer and the absorptive elliptical polarizing plate may be laminated to each other either directly or through an isotropic intermediate layer. Preferably, the refractive index of this intermediate layer is intermediate between the refractive index of the cholesteric layer and that of the absorptive elliptical polarizing plate.
According to the present invention, the cholesteric layer that selectively reflects either one of right-handed and left-handed circularly polarized components of incident light, and the absorptive elliptical polarizing plate that transmits elliptically polarized light containing as its main component the same circularly polarized component as that selectively reflected by the cholesteric layer are laminated to each other, so that the direction of the major axes of liquid crystalline molecules aligned on the absorptive-elliptical-polarizing-plate-side surface of the cholesteric layer can form, with the direction of the major axis of elliptically polarized light produced by the absorptive elliptical polarizing plate, an angle that falls in a specified angle range centering around 90xc2x0. Therefore, of the main circularly polarized component of the elliptically polarized light produced by the absorptive elliptical polarizing plate, the circularly polarized component that has passed through the cholesteric layer by a certain percentage is effectively extinguished when it passes through the displaying element such as a liquid crystal displaying element and through the elliptical polarizing plate provided on the observer side, whereby the quality of black indication is improved. An optical display device in which such a polarizing element is incorporated can thus have enhanced contrast, and, as a result, improvement in displaying quality can successfully be attained.